In such extruders, in which the polymeric material fed in through a hopper is plasticized and masticated by one or more feedscrews prior to being discharged under pressure through a shaping die, the extruder housing is generally heated by external means (electronically or fluidically) to fuse the incoming granules or other particles into a coherent viscous mass. Such fusion advantageously takes place already in a preheating zone located between the housing inlet and the masticating feedscrew or feedscrews.
For maximum efficiency it is, of course, desirable to advance the incoming material through the preheating zone at a rate which, on the one hand, closely approaches the operating capacity of the masticating and discharging mechanism and, on the other hand, allows the entire mass to reach the correct temperature upon passing through that zone. The feed rate and/or the preheating temperature may require modification upon a changeover to a different type of material.
A previously proposed device for such extruders comprises several dished and coaxially superposed heating plates whose bottoms are provided with relatively offset apertures and are individually scraped by bladed wheels rotatable about their common axis to let the material successively pass through the several heating stages constituted by these plates. Similar multistage heaters are known in the field of metallurgy for the roasting of sulfidic ores with or without chlorination.
Such an assembly of stacked heating plates has a relatively limited throughput rate and must therefore be of large dimensions to handle a substantial quantity of material to be treated. If one stage becomes defective, the entire stack must be disassembled to enable its replacement. Since the material passes through the stack only once, heating to high temperatures requires a large number of stages.